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Scientists are unsure how long it takes for the fungus to manifest itself as WNS in bat populations

A fungus that leads to a deadly disease that has killed almost seven million bats in the US is continuing its spread westwards, results have shown.

Officials said the disease had been confirmed in Arkansas after samples tested positive for the fungus known to cause white-nose syndrome (WNS).

To date, there is no known vaccine or antidote against the disease.

WNS was first detected in New York State in 2006 and has since spread to 22 states and five Canadian provinces.

The latest positive results came from swab samples taken from hibernating bats in two cave in Arkansas.

The testing was part of a national study, funded by the US National Science Foundation, being carried out by researchers from the University of California Santa Cruz and North Arizona University.

"These are pretty far west occurrences," explained Ann Froschauer, WNS spokeswoman for the US Fish and Wildlife Service (FWS).

"We do have one potential site further west in western Oklahoma but these latest cases are the most western confirmed cases to date."

Early warning

She told BBC News that the positive results in Arkansas, taken from two caves, only indicated the presence of the fungus, Geomyces destructans, that is known to cause white-nose syndrome but there was no sign of the disease itself in the caves' bat population.

White-nose syndrome

WNS is associated with a fungus known as Geomyces destructans

Once present in a colony, WNS can wipe out the entire population

The disease, which appears to target hibernating species, was first reported in a cave in New York in February 2006

The most common visible symptom of an infected bat is a white fungus on the animal's nose, but it can also appear on its wings, ears or tail

Other symptoms include weight loss and abnormal behaviour, such as flying in daylight or sub-zero temperatures

"As we have got better at the science side of things, we have been able to develop more sensitive [tests] that can detect the fungus in the environment in the absence of sick bats," Ms Froschauer said.

"We have seen that there seems to be some sort of timeline from when the fungus arrives in an area and when we start seeing the disease start manifesting itself in the bat population."

Officials hope data gathered by studies this could help improve their WNS management plans and, ultimately, provide an insight on how it would be possible to disrupt the disease cycle.

Biosecurity measures, such as closing caves to the public and offering decontamination guidance to cavers, are the only method currently available to slow the spread of the disease.

To date, no vaccine exists that can be used to protect uninfected bat populations.

"It is something that we are interested in and there are scientists that are interested in the potential of developing a vaccine," Ms Froschauer observed.

"But this will have to be something completely novel. Mammals do not respond to fungal infections in the same way that they respond to bacteria or viruses.

"Our bodies do not develop antibodies in the same way as it does to bacteria or viruses, so some creative thinking is going to be required."

"If we got to the stage where we had a vaccine then we would have to move on to the challenge of how would we administer it to a small, flying mammal."

'Race against time'

When it came to individuals already infected with WNS, she said that there were recorded cases of bats surviving and appearing to beat the disease.

But it raised more questions than answers, such as whether it affected their ability to reproduce or their ability to survive the following season.

"The bat's body is unlikely to be building up immunity to the fungal disease, so when they come back to the cave in the following year then does seem likely that they could develop the infection again," she suggested.

In order to answer some of those questions, Ms Froschauer said that researchers were embarking on a project to fit small ID bands on individual bats in order to track them.

Overall, she said, there were reasons to be optimistic about the progress scientists were making in terms of understanding the disease's dynamics.

But added that it was "a race against time".

"The science is moving very quickly but the disease seems to be moving a little bit quicker."